Attending the White House event were a group of families with children who were born from "adopted" frozen embryos that had been left unused at fertility clinics.

"These boys and girls are not spare parts," he said of the children in the audience. "They remind us of what is lost when embryos are destroyed in the name of research. They remind us that we all begin our lives as a small collection of cells."

The measure, which the House of Representatives passed in May 2005, allows couples who have had embryos frozen for fertility treatments to donate them to researchers rather than let them be destroyed.

Bush said, "If this bill were to become law, American taxpayers would, for the first time in our history, be compelled to fund the deliberate destruction of human embryos, and I'm not going to allow it."

In August 2001, Bush announced that his administration would allow federal funding only for research on about 60 stem-cell lines that existed at the time. Researchers have since found that many of those lines are contaminated and unusable for research.

The Senate bill's principal sponsor, Sen. Arlen Specter, R-Pennsylvania, who recently survived a brush with cancer, was joined by Majority Leader Bill Frist, R-Tennessee, a physician who argued that Bush's policy is too restrictive.

"I am pro-life, but I disagree with the president's decision to veto the Stem Cell Research Enhancement Act," Frist said in a statement. "Given the potential of this research and the limitations of the existing lines eligible for federally funded research, I think additional lines should be made available."

Also in a statement, Lawrence T. Smith, chairman of the American Diabetes Association, called the veto "a devastating setback for the 20.8 million American children and adults with diabetes -- and those who love and care for them."

Opponents argue that other alternatives, such as adult stem cells, are available. Two companion bills -- one to promote alternative means of developing stem-cell lines from sources such as placental blood and another to ban the commercial production of human fetal tissue, also known as "fetal farming" -- passed the Senate in 100-0 votes.

On Tuesday evening, the House approved the "fetal farming" bill 425-0 but didn't pass the measure promoting alternative stem-cell sources when backers failed to achieve the two-thirds majority that House rules required. The vote on the alternative-sources bill was 273-154.

Bush signed the "fetal farming" legislation and urged Congress to fund alternative research.

"I'm disappointed that the House failed to authorize funding for this vital and ethical research," he said. "It makes no sense to say that you're in favor of finding cures for terrible diseases as quickly as possible and then block a bill that would authorize funding for promising and ethical stem-cell research."

A House GOP aide said that the leadership would bring the funding bill back to the floor at another time under a different set of rules that would require a simple majority to pass the measure.